Thermionic valve amplifier circuit arrangement



P 1944. L. 0. WHITE 2,358,428"

THERMIONIQ VALVE AMPLIFIER CIRCUIT ARRANGEMENTS Filed May 15, 1945INVENTOR iii. a W/ui WW 1 BY wzm ATTORN EY Patented Sept. 19, 1944THERMIONIC VAL AMPLIFIER CIRCUIT VE RANGEMENT Eric Lawrence CaslingWhite. Chlswick, London,

England, asslgnor to Electric & Musical Industries Limited, Hayes,Middlescx, England, a

company of Great Britain Application May 15, 1943, Serial No. 487,160 InGreat Britain September 7,1940

6 Claims. (Cl- 179-471) This invention relates to cathode follower cirrangements including a valve having a load connected in its cathodecircuit in such a manner as to provide negative feedback into its gridcircuit so that the apparent output impedance of the cathode circuit ofsaid valve as seen by said load is low and of the order of the inverseof the mutual conductance of said valve.

One of the objects of the present invention is to provide a circuitarrangement employing a pair of valves, one of which is arranged in acathode follower circuit, for affording what is efiectively a push-pulloutput without the necessity of employing a push-pull output transformersuch as is usually required with push-pull circuits.

According to the invention, a circuit arrangement is provided comprisinga thermionic valve and a load arranged in a cathode follower circuit,means for applying signals to the control electrode of said valve, afurther valve having its output circuit arranged to feed current to saidload in opposite sense to the current fed thereto by saidfirst-mentioned valve, said further valve also being arranged to becontrolled by said signals, the arrangement being such that if the cuitarrangements, that is to say, circuit arate on reasonably straightportions of their characteristic curves. By so biasing the valves it canbe arranged that for large positive excursions of input signal voltagesthe load current is almost entirely supplied via one of said valves andfor large negative excursions the load current is almost entirelysupplied via the other valve. By employing two valves biased in theabove manner, the mean current can be substantially reduced ,comparedwith the mean current which would be required for a single valveoperating in a cathode follower circuit providing the same power output.

Preferably, the control electrode of said further valve is coupled to animpedance associated with the valve in the cathode follower circuit insuch a manner that if signals are applied to said valve signals are alsoapplied to said further valve but in opposite phase to the signalsapplied to said valve. The impedance is preferably provided in the anodecircuit of said valve. An impedance may be connected in the cathodecircuit of said further valve so as to provide negative feedback tolinearize said further valve.

In order that the invention may be clearly understood and readilycarried into effect it will now be more fully described with referenceto the accurrent flowing through said load under the control of saidfirst-mentioned valve is caused to increase on the application of saidsignals the current flowing through said load under the control of saidfurthervalve is caused to decrease wd vice versa.

The output power which can be delivered to the load by a cathodefollower circuit arrangement is limited by the available cathode currentswing of the valve and it is usual to choose a valve capable of passing,without overheating, a mean current at least equal to one-half of therequired output current swing and a peak current at least equal to therequired current swing. One of the disadvantages of employing a valve inthis mannet is that the mean power output for a given size of valve islimited to a fraction of the maximum power output of the valve. Afurther disadvantage is that the mean current drawn by companyingdrawing, in which:

Figure 1 illustrates a circuit arrangement according to one embodimentof the invention, and

Figure 2 illustrates a furtherembodiment of the invention.

Figure 1 of the drawing illustrates the invention as applied to anoutput stage of an amphfler feeding a load in the form of a lowimpedance concentric cable represented by the resistance I. The triodevalve 2 has the resistance I connected in its cathode circuit so as toprovide negative feedback in its grid circuit, and the arrangementfunctions as a cathode follower circuit in which the impedance seen bythe load is low and said valve is substantial and the arrangement istherefore uneconomicala By employing the ar-' rangement according to theinvention, the available power output of the valve can be more fullyutilized. The mean current can also be reduced by so biasing said valvesthat in the absence of signals the anode current of each valve is small.The biases are, however, preferably arranged to be sufflcient to ensurethat the valves will operis substantially equal to the inverse mutualconductance of the valve 2. The cathode of the valve 2 is connected tothe anode of a further valve 3, the control electrode of which lattervalve is coupled to the resistance shown in the anode circuit of thevalve 2-via the coupling capacity 4 and is also connectedto the cathodeof the valve 3 via the leak resistance shown. The cathode of the valve 3is connected via the source of anode current, represented conventionallyby a battery, to the negative terminal of the source of anode currentfor the valve 2, to which the lower end of resistance l is alsoconnected.

Thus, when signals are applied to the input in opposite sense to thecurrent fed through the load by the valve 3. When the control electrodeof the valve 2 is made positive on the application of the signals, theanode current of the'valve 2 increases and the anode potential of thevalve 2 decreases. This decrease of anode potential is applied via theresistance-capacity coupling to the control electrode of the valve 3 sothat the anode current of the valve 3 is reduced. On the application ofa signal causing the control electrode of the valve 2 to become lesspositive, the anode current of the valve 2 diminishes the increase inanode potential, which result is communicated to the control electrodeof thevalve 3 so that the anode current of this valve increases.

Th bias potentials applied to the control electrodes of the valves 2 and3 are so chosen that in the absence of input signal voltage the anodecurrent of each valve is small, but suflflcient to insure that thevalves will operate on reasonably straight portions of theircharacteristic curves. It can therefore be arranged that for largepositive excursions of the input signal voltag the current flowingthrough the load I is almost entirely supplied via the valve 2; and forlarge negative excursions of input signal voltage the load current isalmost entirely supplied via the valve 3. The biases should be so chosenin relation to the signal voltages that if the anode current of thevalve 2 were reduced to zero the anode potential of the valve 2 would beraised suiliciently to causethe valve 3 to pass maximum current. Inoperation, however, the bias potential applied to the valve 2 should beso chosen that the anode current of this valve is never entirely reducedto zero 50 that the output impedance always remains low. The output im-,pedance of the two valves combined as seen by the load I can, ifdesired, be arranged to be less than the inverse of the mutualconductance of the valve 2.

If the signal voltages applied to the valve 2 comprise a televisionsignal of the usual waveform consisting of picture signals andsynchronising signals in different amplitude ranges, it can be arrangedthat the picture signal output is supplied mainly by current from thecathode follower valve 2 and the synchronising signal output is suppliedmainly by current from the valve 3.

The circuit shown in Figure 1 thus provides what is effectively apush-pull output without the necessity of employing a push-pull outputtransformer and enables a substantial economy to be effected comparedwith a cathode follower circuit having a single valve producing the samepower output.

The arrangement of Figure 2 shows an output stage feeding a capacityload 5, such as the modulating electrode of a cathode ray tube, which isof low impedance at high signal'frequencies. As in the arrangement ofFigure 1, it will be seen in Figure 2 that the cathode of the valve 2 inthe cathode follower circuit is connected directly to the load and tothe anode of the valve 3, the control grid of which is coupled throughthe condenser 4 and leak resistance to the anode resistance of the valve2. VA resistance is shown connected in the cathode lead of the valve 3,the value of the resistance being selectedto provide negative feedbackfor the valve 3 such that it will operate substantially linearly. Sincethe output voltage swing is likely to be large, pentode type valves areemployed instead of triodes which are shown in the arrangement of Figure1, in

which the voltage output swing is low. The oper'- ation ofthearrangement is in substance similar to that already described withreference ure 1.

This application of the invention permits great economy in anodecurrent, as high currents are only drawn when sudden changes of inputsignal voltages occur necessitating sudden bursts of current to chargeor discharge the capacity 5.

Although the invention has been described with reference to outputstages delivering television signals to low impedance loads, it will beunderstood that other kinds of signals may be used and the invention isparticularly useful when the signals consist largely of sharp pulses. 4

I claim:

1. In an amplifier system of the cathode follower type comprising anelectron discharge tube provided with signal input terminals and havinga load impedance element in the cathode circuit thereof; the improvementwhich comprises an electron discharge device having at least a controlelectrode, cathode and anode, the anode to cathode path of said deviceand a current source being connected .in series between the cathode andplate of said tube, means applying signal. voltage existing at saidlatter plate to said control electrode, said load element beingconnected in shunt relation to said anode to cathode path, and thecathode of said tube and anode ofsaid electron discharge device being ata common divoltage existing at said latter plate to said controlelectrode, said load element being connected in shunt relation to saidanode to cathode path,

and the cathode of said tube and anode of said electron discharge devicebeing at a common directcurrent potential, an unbypassed resistiveimpedance in the space current path of said device for providingdegenerative feedback to its said control electrode.

3. In an amplifier system, a pair of electron discharge devices eachhaving at least a cathode, control electrode and output electrode, aclosed circuit including the space current paths of said devices inseries relation, means establishing the output electrode of one deviceat a positive potential relative to the cathode of the second device, aninput circuit connected between the control electrode of said one deviceand a point of reference potential, nected between the cathode of saidone-device and said point of reference potential, means for to manoutput circuit con- 7 trode oi the second device to the cathode-oi thefirst device. v

4. In an amplifier system, a. pair of electron discharge devices eachhaving at least a cathode, control electrode and output electrode, aclosed circuit including the space current paths of said devices inseries relation, means establishing the output electrode of one deviceat a positive potential relative to the cathode of the second device, aninput circuit connected between the control electrode of said one deviceand a point of reference potential, an output circuit connected betweenthe cathode of said one device and said point of reference potential,means for coupling the control electrode of the second device to theoutput electrode ofthe first device, and means directly connecting theoutput electrode of the second device to the cathode of the firstdevice, a resistive load-element in' said output circuit, said loadelement being unbypassed to provide degenerative feedback to said inputcircuit.

5. In combination withla signal amplifier tube having input and outputelectrodes, a resistive load element in the tube space current pathcommon to the input electrodes and output electrodes of the tube wherebysignal voltage developed across the load element is applied between thetube input electrodes, means for applying signals to said inputelectrodes, a second tube provided with input signal connections to theoutput electrodes of said amplifier tube, and connections including saidload element between the output electrodes of said second tubewherebysaid load element is arranged in push-pull relation to the space currentpaths of said two tubes.

6. In combination with a signal amplifier tube having input and outputelectrodes, a resistive load element in the tube space current pathcommon to the input electrodes and output electrodes of the tube wherebysignal voltage developed across the load element is applied between thetube input electrodes, means for applying signals to said inputelectrodes, a second tube provided with input signal connections to theoutput electrodes of said amplifier tube, and connections including saidload element between the output electrodes of said second tube wherebysaid load element is arranged in push-pull relation to the space currentpaths of said two tubes, and adirect current connection from the anodeof the first tube to the cathode of the second tube including a directcurrent source and a resistor in

